That number comes courtesy of the Seattle Post-Intelligencer, which did a little digging into Microsoft's annual U.S. Securities and Exchange Commission report. It comes from subtracting Xbox 360-related revenue -- a cool $8.1 billion -- from that of its overall Entertainment and Devices Division, leaving $613 million split between Windows Phone and a variety of much smaller projects such as Zune, Surface and the thoroughly out-to-pasture Windows Mobile platform.

Microsoft rolled out Windows Phone in late 2010, meaning that any revenue numbers aren't reflective of a full year of sales. Nonetheless, sales totaling under $613 million would be anemic compared to those of Apple's iPhone over a similar period, or even the combined family of Google Android devices.

Microsoft habitually refuses to release any hard sales data related to Windows Phone. Nonetheless, studies from various research firms have suggested Microsoft holds a single-digit share of the U.S. smartphone market. Research firm comScore, for instance, placed Microsoft at 5.8 percent by the end of May, trailing Android at 38.1 percent, Apple at 26.6 percent and Research In Motion's BlackBerry franchise at 24.7 percent.

Microsoft occupied some 9 percent of the U.S. smartphone market in June, according to data from Nielsen. That total, which included the company's Windows Mobile platform, trailed Google Android with 39 percent, Apple's iPhone with 28 percent and RIM with 20 percent.